1. Field of the Invention
The present invention relates to a tilt servomechanism for correcting tilt errors of an optical beam irradiated onto an optical disk.
2. Description of the Prior Art
In general, in using optical disks including CDs or DVDs, warps of such optical disks causes the problem that a tilt angle made between the axis of an optical beam emitted from an optical pickup and the disk surface deviates from a right angle at a beam-irradiated position. The deviation is known as xe2x80x9ctilt error.xe2x80x9d The tilt error occurs mainly in the radial direction of optical disk, resulting in coma aberration of optical systems. Moreover, the tile error will cause crosstalk between adjacent tracks or deterioration of jitters, thereby bringing about an undesirable effect on reproduction quality of the optical disk. Particularly, if information is recorded at a high density like DVD, a laser beam is required to be smaller in its spot diameter. Namely, it is necessary to shorten the wavelength xcex of a laser beam as well as to increase the numerical aperture NA of an objective lens. This results in fewer margins for the tilt angles. Therefore, a slight tilt of an optical disk causes a greater degree of deterioration in reproduction quality. To correct aberration caused due to inaccurate tilt angles during the reproduction of information, a tilt servo control is normally performed by using both a tilt sensor with an optical beam dedicated to detecting the tilt angle and detected by a detector and a mechanism for controlling the inclination of the optical pickup based on the detected tilt angle.
However, in the above tilt servomechanism, the tilt sensor having a pair of optical beams and a detector should be provided independently of the optical pickup, and a complex mechanism is required to control the inclination of the optical pickup. As a rule, these factors result in the increased manufacturing cost. Additionally, they disadvantageously occupies a large volume of space for installation, thereby disabling the downsizing of the apparatus.
The present invention has been made in consideration of the above problems. An object of the present invention is to provide a tilt servo control apparatus for an optical disk, which is able to keep a high reproduction quality by correcting the tilt error with simple and compact configuration.
According to one aspect of the present invention, there is provided a tilt servo control apparatus including: an optical pickup for irradiating an optical beam onto an optical disk and detecting a reflected light from the optical disk; a tilt-correction unit for correcting, using a correction amount, a tilt error due to aberration caused by a tilt angle formed between an optical axis of the optical beam and a normal line of the optical beam at an emitted position on the optical disk; an amplitude detecting unit for detecting a maximum amplitude and a minimum amplitude of a detected signal by the optical pickup within a period of one rotation of the optical disk; and a tilt-correction controlling unit for providing a control signal to control the correction amount used in the tilt-correction unit, the tilt-correction controlling unit including a unit for applying variations to the control signal and adjusting the control signal so that a difference between the maximum amplitude and the minimum amplitude is minimized responsively to the variations.
In the above apparatus, even when a tilt angle varies and the aberration of the optical pickup is caused, the aberration is corrected by applying variations to the control signal and minimizing the changes in the amplitude of the detected signal from the optical pickup. When the tilt error is caused, the variations in the amplitude are increased, thereby deteriorating the detection characteristic of the optical pickup, however, the above tilt control can compensate for the error. Therefore, deterioration of reproduction quality, which is caused by tilt errors, can be avoided. The tilt servo control can be performed with no tilt sensor.
It is preferred that the tilt-correction unit includes a liquid crystal panel for correcting the aberration arranged in the optical axis of the optical beam, and the control signal used in the tilt-correction controlling unit is a drive signal applied to the liquid crystal panel. It is also preferred that the tilt-correction controlling unit includes a unit for producing disturbances to be applied to the control signal, a unit for comparing differences between the maximum amplitude and the minimum amplitude corresponding to the disturbance, and a unit for varying the control signal by a given amount such that the differences are reduced.
Thus, the disturbances is added to the control signal in such a way that the difference between the maximum value and the minimum value of the detected signal of the optical pickup decrease. Therefore, like the above, the reproduction quality of an optical disk can be avoided from being deteriorated even when tilt error is caused.
Preferably, the tilt-correction may include a liquid crystal panel for correcting the aberration arranged in the optical axis of the optical beam, and the control signal used in the tilt-correction controlling unit is a drive signal applied to the liquid crystal panel. Thus, with the liquid crystal panel for correcting aberration caused in the optical beam axis, the tilt error is corrected. In consequence, the identical advantages to the above are obtained and the tilt servo control can be done with no mechanism for controlling the inclination of the optical pickup.
In a preferred embodiment, the liquid crystal panel may be divided into three areas consisting of an inner-track-side area, a middle area, and an outer-track-side area in a radial direction of the optical disk, wherein the divided areas are driven by the drive signal independently of each other. Therefore, it is possible to advantageously prevent deterioration of reproduction quality and eliminate a tilt sensor and a mechanism for controlling the inclination of the pickup.
It is preferred that the apparatus further includes temperature detecting unit for detecting a temperature relating to operation of the liquid crystal panel, wherein the tilt-correction unit includes a unit for variably controlling a speed of the variations applied to the control signal on the basis of a result detected by the temperature detecting unit. Accordingly, the temperature detecting unit are used for detecting a temperature of, for example, the liquid crystal panel. In response to the result of the temperature detection, the rate of variations in the control signal is made changed. Thus, if the response characteristic of liquid crystal is deteriorated on account of fluctuations in temperature, the tilt correction can be performed in a highly stable manner. The tilt servo control can be done with preferable temperature characteristics.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiment of the invention when read in conjunction with the accompanying drawings briefly described below.